1. Field of the Invention
The present invention relates to high-frequency modules provided in antenna front-ends of wireless communication devices such as cellular phones and to communication devices each including a high-frequency module and a receiver that is separately provided in addition to the high-frequency module.
2. Description of the Related Art
FIG. 5 is a circuit diagram of a high-frequency module according to an exemplary prior art (for example, see Japanese Unexamined Patent Application Publication No. 2007-129459) to be used in an antenna front-end of a cellular phone and the like. A high-frequency module 101 includes a switch IC 102, a duplexer 103, and a duplexer 104. The high-frequency module 101 has the capability of transmitting and receiving signals in a first communication band and a second communication band using a transmission/reception antenna Ant1. The switch IC 102 includes an antenna connection port PsA1 and two switch ports Ps1 and Ps2, and has the capability of switching connection between the antenna connection port and the switch port. The duplexer 103 is connected to the switch IC 102 via the switch port Ps1, and including a transmission filter and a reception filter for signals in the first communication band. The duplexer 104 is connected to the switch IC 102 via the switch port Ps2, and including a transmission filter and a reception filter for signals in the second communication band.
In this high-frequency module 101, inputting a transmitting signal in the first communication band or the second communication band causes the switch IC 102, the duplexer 103 or 104 to generate a second-order harmonic or a third-order harmonic of the transmitting signal, and these harmonic signals travel to the antenna Ant1.
For example, when a transmitting signal Tx at a fundamental frequency fo is inputted to the high-frequency module 101 via the duplexer 103, a harmonic of the transmitting signal Tx is generated at the switch IC 102 or the duplexer 103. In the case where the second-order harmonic with frequency 2fo is generated at the switch IC 102, a second-order harmonic Hm1 with the frequency 2fo is outputted from the antenna connection port PsA1 of the switch IC 102, and a second-order harmonic Hm2 with the frequency 2fo is outputted from the switch port Ps1 of the switch IC 102. The duplexer 103 is set up in such a way that its impedance matches with that of the switch IC 102 at the fundamental frequency fo of the transmitting signal Tx1. Thus, at the frequency 2fo, the matching with the switch IC 102 is not achieved, and the second-order harmonic Hm2 is mostly reflected. The second-order harmonic Hm2 reflected on the duplexer 103 is outputted to the antenna Ant1 together with the second-order harmonic Hm1 via the switch IC 102. Accordingly, the antenna Ant1 for use in transmission transmits the second-order harmonic (Hm1+Hm2) at the frequency 2fo in addition to the transmitting signal Tx at the fundamental frequency fo.
As described above, in the high-frequency module of prior art, the harmonics generated at harmonic sources such as the switch IC and the duplexer travel to the antenna together with the transmitting signal. In the case where a frequency band of the harmonic is used by another communication system, this may pose a problem of lowering reception sensitivity and the like of another communication system. For example, Band 13 of Long Term Evolution (LTE) system has a transmitting signal band from 777 MHz to 787 MHz, and its second-order harmonic band is from 1554 MHz to 1574 MHz. This second-order harmonic band is very close to the band (from 1574.42 MHz to 1576.42 MHz) used for Global Positioning System (GPS). Thus, it is difficult to obtain sufficient attenuation of the second-order harmonic of LTE-Band 13 with a reception filter provided in a GPS receiver. Accordingly, in the case where cellular phones and the like use LTE-Band 13 and include GPS receivers, the second-order harmonic of LTE-Band 13 may reach the GPS receiver and reduce the reception sensitivity of the GPS receiver.
Thus, it is desirable that the harmonic transmitted from the antenna is stable and at a low signal level. However, in some cases, the signal level of the harmonic transmitted from the antenna becomes higher in the high-frequency module of prior art. For example, in the case where a harmonic that directly traveled to the antenna from the source of harmonic such as the switch IC coincides in phase with a harmonic that traveled to the antenna after being reflected at a harmonic reflector that reflects harmonics such as the duplexer, the signal level of the harmonic transmitted from the antenna may become higher due to addition of their signal levels.
Further, in the case where the harmonic source generating harmonics or the harmonic reflector reflecting harmonics is connected as an external circuit, the electrical length of a signal path for a harmonic that travels from the external circuit to the antenna varies depending on the configuration of the external circuit. Thus, the phase of the harmonic that travels from the external circuit to the antenna varies depending on the configuration of the external circuit. On the other hand, the signal path and phase are constant in the harmonic that travels the inside of the high-frequency module and arrives at the antenna. Accordingly, the signal level of the harmonic transmitted from the antenna, namely, the harmonic characteristics of the high-frequency module vary according to the configuration of the external circuit due to the addition of harmonic signals that are different in phase.